Improve Venting/Drainage Times and Avoid Floating
How Can Venting/Drainage Times Be Optimized?
For successful batch hot-dip galvanizing, cleaning solutions and molten zinc must flow without undue resistance into, over, through, and out of the fabricated article. Failure of the customer to provide for this unimpeded flow can result in bare spots, weld blowouts, and excessive build-up of zinc. Additionally, poor design negatively impacts the safety of plant personnel and hurts the galvanizer’s bottom line when longer dip and withdrawal times are combined with excessive clean-up of drips and runs. The necessary details for venting and drainage holes are provided in ASTM A385, Practice for Providing High-Quality Zinc Coatings (Hot-Dip). When a customer does not provide suitable quantity, sizes, and locations of holes according to the specification, the galvanizer is often left to work with the customer to provide recommendations based on experience and at minimal cost. However, it can be difficult to determine optimal venting and drainage design for long and/or complex structures.
Below is a summary of the factors that affect fill and withdrawal times, and example charts to assist in demonstrating the minimum vent/drain hole requirements for optimizing quality and safety while minimizing production cost.
The primary factors that have the greatest impact on immersion times in the galvanizing bath are:
- Part Dimensions (length, width, and height or length and diameter) - affect the amount of zinc that must be displaced by the part in the bath, and the flow within the part. The more volume displaced, the more buoyant force works on the part.
- Vent/Drain hole size – increased hole sizes improve flow rates to fill and drain the part.
- Dip angle – combined with the part length, the angle affects the lifting height during immersion, and therefore the distance molten zinc needs to move vertically up the part.
- Floating Potential: if too much air remains in the part during immersion in the kettle due to poor placement and/or undersizing of vent/drain holes relative to the part size, the entrapped air can provide enough of a buoyant force to cause floating, making the part difficult or impractical to immerse.
To give you an example of how quickly air entrapment can lead to floating with some general bath and part properties, Figure 2 shows the amount of entrapped air that causes standard pipe sizes to float within the zinc bath.
When galvanizing smaller pipe (6-in nominal or less) significant air entrapment is required for floating – about 20-50% air. Meanwhile, less than 10% air entrapment leads to floating in 14-in pipe, and 36-in pipe can float with only 3% air. As you can see, hole placement becomes critical with complex, long, or large hollow parts.
Figures 3 and 4 describe the impact of vent and drain hole sizes on three 30-ft pipes of diameters 36-in, 18-in and 6-in. Supplying one large 2-in hole (or 12.5 in2 area), a 6-in pipe would fill in 1min and drain in 30 seconds, but the same hole design results in a 15 minute fill/3 minute drain for the 18-in pipe, or over 30 minute fill/13 minute drain for the 36-in pipe.
Secondary factors impacting immersion and withdrawal times are:
- Fluidity of the Zinc - Increased bath operating temperature and the use of bath additives such as aluminum, bismuth, and/or lead improve the fluidity of the zinc, resulting in improved drainage.
- Roughness of Holes – rougher (flame-cut or torched) holes disturb the flow and can sometimes noticeably decrease flow rates, or cause a different type of flow. Furthermore, rough edges make measuring the hole difficult and often result in undersized holes. Drilled holes and/or smoothed holes are strongly preferred to improve flow.
- Hole Shape - wide and short allows greater flow rates and decreases the possibility for floating. However, round holes are more easily filled with plugs after galvanizing.
- Geometry of the hollow section – circular products fill and drain more quickly than square or rectangular products of equivalent size.
- Minor Losses – a technical term for changes in the flow direction (elbow, tees) or changes in the cross-sectional area (or diameter) the zinc flows through. Practically, this means the more complex the hollow fabrication, the longer the immersion/withdrawal time.
Analyzing the primary and secondary factors will allow you to develop a customized recommendations for any customers looking to improve venting and drainage outside the established guidelines.
If you must negotiate alternative venting and drainage designs with the customer, it is advantageous to get any and all changes in writing, have the customer sign a waiver to note potential aesthetic and quality concerns, and address the amount of additional labor required for cleanup in a change order.
© 2020 American Galvanizers Association. The material provided herein has been developed to provide accurate and authoritative information about after-fabrication hot-dip galvanized steel. This material provides general information only and is not intended as a substitute for competent professional examination and verification as to suitability and applicability. The information provided herein is not intended as a representation or warranty on the part of the AGA. Anyone making use of this information assumes all liability arising from such use.